1. Field of the Invention
This invention relates to encoders and more particularly to improved matrix and detecting circuit arrangements for encoders including arrays of resistive sensors.
2. Description of Prior Art
Electrical control and measuring apparatus usually are required to monitor a physical condition in order to perform an intended operating function. Typically, predetermined command functions, status reporting functions or measurement functions are produced in response to the monitored conditions. Electronic encoders are included in the electrical apparatus to generate analog or binary coded output signals responsive to the changes in the status of the monitored condition. The encoder output signals are either used at the apparatus or transmitted via a telemetry communication system to remote equipment. Generally, the encoders include a transducer or sensor unit, a detecting unit and a readout unit. The transducer unit typically includes a network of several electrical sensors having variable voltage, current, impedance, frequency or phase angle sensing states responsive to the monitored condition. The detecting unit includes both sampling control and detecting circuits to sample the sensing states of the sensors and produce corresponding detection signals. In turn, the readout unit converts the detection signals into coded output signals representing the status information in a desired form and format.
Various types and forms of encoders are known for producing coded data outputs in response to one or more inputs. The different encoder types are chiefly determined by the different types of input and output arrangements thereof and the various modes of input and output operation. When the encoders include large arrays of sensor or encoding elements, the encoder arrangements and operations often become somewhat complex and complicated.
One general mode of operation in encoders includes determining, on a one at a time basis, the status at each individual sensor element of a large array. Examples of this one general mode of operation are disclosed in U.S. Pat. Nos. 4,037,219, issued July 10, 1979, and 4,137,451, issued Jan. 30, 1979, both assigned to the assignee of this invention. Several arrangements are known for scanning or sampling array sensor elements and detecting the status thereat. For example, a single encoder input can be arranged to sequentially examine many sensor elements by being physically displaceable relative to the elements or by having a means for coupling each of the elements to the single input. Complexities of mechanical design or the space required for such single input arrangements may, however, prevent their use. In another arrangement, separate inputs to each of the array sensor elements permit individual monitoring of each element or group of elements in a parallel fashion. Simultaneous parallel encoding of many sensing elements does reduce the time otherwise required for sequentially sampling and encoding the same number of elements. Often it is undesirable to have a large number of encoder inputs, as would be required for a large array of sensors, and especially undesirable to have large numbers of both inputs and outputs when separate outputs are also provided for each element.
Further encoder arrangements are known to reduce the number of encoder inputs and outputs and to simplify the input and output scanning and detecting operations. In particular, sensor elements of large circuit networks can be interconnected to reduce the number of inputs and outputs. When the sensor elements are connected in a network having the row and column organization of a rectangular matrix network, the number of element interconnections can be reduced as well as having the number of associated inputs and outputs reduced. It has been found, however, that selecting a specific matrix configuration for an encoder array involves several and often conflicting considerations. Some critical considerations especially occur when the elements of a large array are of a variable impedance type and are interconnected by common row and column conductors in a matrix circuit arrangement. For example, resistive sensors form one type of variable impedance element that can be interconnected in such a matrix circuit arrangement, but their resistance values can produce multiple diverging branch paths and interfering series paths within the matrix. Thus, scanning of a row input and a column output associated with a preselected resistive sensor may not provide a unique sensing state at the output. Still further, the output sensing state is particularly affected when the other resistive sensors of the array can have different and varying resistances at any instant.
Examples of arrays having a resistive sensor type of encoding element are described in the aforementioned U.S. Pat. Nos. 4,037,219 and 4,137,451 wherein the arrays specifically include photoconductive type resistive sensors. The photoconductive sensors have resistance values which vary in response to dial shaft encoding patterns of illumination. The photoconductive sensor arrays in the two aforementioned patents are connected in a circuit network having separate input terminals which are sequentially scanned. Voltage sensing signals are developed at a single output for detection of the sensor resistance values by an analog voltage comparator detecting circuit. The separate scanning inputs of the aforementioned encoder arrangements substantially aid in isolating the sensor resistance values to be detected in an array, however, for arrays having large numbers of sensors, it is sometimes unduly complex to provide a correspondingly large numbers of inputs to scan each sensor.
U.S. Pat. Nos. 3,806,875 and 3,662,368 disclose further examples of encoders including photocell arrays arranged to be responsive to the encoding of meter dial readings. The conductive and nonconductive states of the photocells are sampled through separate input leads to generate coded readout signals for remote meter readings. The last two named patents have arrangements of separate scan inputs and separate sensing outputs for each different sensor and such arrangements have substantially increased complexity when used with large arrays.
In U.S. Pat. No. 3,573,773, a meter dial encoder readout device is disclosed having an array of photoconductive-type photocell sensors connected in a network with additional diodes and resistors to incrementally vary the total resistance of the network. Although the encoder arrangement avoids sampling at multiple scan inputs, the encoder operation is limited by activating only two sensors at any one instant while requiring diodes to provide current isolation between the sensors.
When providing separate scanning inputs to array networks such as included in the encoders described above and having a large number of sensor elments, one specific difficulty is in providing a circuit, referred to herein as a sampling control, to produce the sequential scanning signals. Circuit devices for sampling control circuits may have a fixed and limited number of input/output terminals, such as provided in some small microprocessor devices, often requiring additional and complex counter, shift register or multiplexing circuits at either or both of the network inputs and outputs. Thus, connecting an array in a matrix circuit arrangement can simplify the sampling control circuits by providing fewer scanning input terminals and sensing output terminals. As is known, the sum of the inputs and outputs of a rectangular matrix arrangement can be substantially less than the number of sensor connected intersections which equals the product of the inputs multiplied by the outputs.
To connect an array in a matrix circuit arrangement to avoid the large encoder input and output arrangements, some of the aforementioned difficulties are found to exist. For example, when the photoconductive sensor array described in the aforementioned U.S. Pat. No. 4,137,451 is connected in a rectangular matrix, rather than in the network disclosed therein, the voltage comparator detecting circuit was sometimes found ineffective to unambiguously distinguish the variable resistance states of each of the photoconductive sensors of the array. One observed difficulty in sampling the individual sensor states is the many parallel and branch current paths that are formed through the resistive sensors being interconnected by the row and column conductors of the matrix. When one matrix configuration is selected having more outputs and fewer inputs than another configuration, the number of possible diverging intercolumn branch circuits increases. The diverging branch current values also vary as different groups of sensors are illuminated and darkened so as to have different encoding resistance states at any given sampling time. Further error producing factors are attributed to variations in the photoconductive sensor characteristics and variations in the encoding levels of illumination and darkness at different sensors. Isolating diodes can be connected to each sensor of a rectangular matrix but this substantially increases the complexity and cost of the matrix. Further difficulty is found in connecting the separate diodes when all of the photoconductive sensors are formed on a common circuit substrate with additional terminals and connections being difficult to form and provide thereon. Also, it is found that in providing separate detecting circuits at each matrix column output, the detecting circuits may also form parts of the branch current paths and further, they must have closely matched characteristics and threshold sensing levels to further avoid ambiguous detection signal outputs.
With the above considerations found in the prior art in mind, the present invention provides an encoder including an improved matrix circuit arrangement for interconnecting a variable resistance sensor array with an improved current responsive detecting circuit being provided for sensing the state of each sensor and producing detection signals having improved accuracies as summarized hereinafter.